| Viktor Troicki retired after the first set of his third-round matach against fellow Serb Novak Djokovic [GALLO/GETTY]
Venus Williams has added her name to the long list of retirements from the Australian Open, quitting after just six points of her match against Andrea Petkovic.
The American, seeded fourth, suffered what she described as the worst pain she has ever felt in her second round match on Friday, and the effects were clearly visible.
After attempting to return a serve from German Petkovic, Williams once again yelped in pain, and retired shortly afterwards.
Next up for Petkovic, the 30th seed, is the only remaining former champion of Melbourne, Maria Sharapova, the 4-6 6-4 6-4 victor over another German, Julia Görges.
The only other former winner in the field was Justine Henin, but the 11th seed bowed out to an improving Svetlana Kuznetsova.
Henin, who suffered a career-threatening elbow injury at Wimbledon last year, clearly struggled with her forehand, and lost 4-6 6-7 to the two-time Grand Slam champion.
Despite the early loss, the Belgian should take positives from her first tournament back, and continue to improve as the year goes on.
Meanwhile, Kuznetsova could produce another shock with victory over French Open champion Francesca Schiavone, who won 6-0 7-6 against Monica Niculescu.
Top seed Caroline Wozniacki had no such trouble against the diminutive Dominika Cibulkova, beating the Slovakian 6-4 6-3. She takes on unseeded Latvian Anatasija Sevastova in round four.
The final fourth-round match in the top half of the draw will pit the eighth and ninth seeds against each other.
Victoria Azarenka will find it tough to beat the in-from Li Na, in what could be a fabulous display of talent.
Roger Federer continued his smooth passage through the men's draw, easing past the resurgent Xavier Malisse 6-3 6-3 6-1.
Although easier than his previous match against Gilles Simon, the Swiss 16-time Grand Slam champion never needed to produce the class of tennis that has seen him regarded by many as the greatest player of all time.
The second seed will have one eye on a quarter final spot, as he takes on Tommy Robredo in round four.
The Spaniard beat fellow unseeded Sergeiy Stakhovsky in four sets.
The winner of that match will play either Andy Roddick or Stanislas Wawrinka.
Roddick, the eighth seed in Melbourne, beat 23-year-old Robin Haase 2-6 7-6 6-2 6-2, whilst Switzerland's Wawrinka blew 12th seed Gael Monfils off the court in straight sets.
One of the most mouth-watering clashes of the fourth round sees Tomas Berdych, runner up at Wimbledon in 2010, take on Fernando Verdasco.
Both hit the ball phenomenally hard, and fireworks are to be expected between these two top-ten players.
Injury claimed yet another player in the third round, with Serbian Viktor Troicki retiring after the first set against fellow countryman Novak Djokovic.
The third-seeded Djokovic had won the first set 6-2 before his Davis Cup compatriot eased his progress into the fourth round.
Djokovic will be the favourite to reach the quarter finals when he plays another Spaniard, Nicolas Almagro, in round four.
With both Williams and Troicki retiring, and Henin, Ivanovic and del Potro still feeling the effects of their injuries, there will be a greater desire to increase the end-of-year break for players.
After all, nobody likes to see anyone quit after six points.